Cathode alloy



Dec. 22, 1942. I E. ca. WIDELL 2,306,290

CATHODE ALLOY Filed Oct. 5, 1941 Oxide coafed i k core confaining .06 f0 .26 percenf silicon, and .10 fa .40 peraen 2* carbon.

INVENZT'OR Emil a. Wide/I.

4 ATTORNEY Patented Dec. 22, 1942 CATHODE ALLOY I Emil Gideon Widell, Bloomfield, N. J assignor to Radio Corporation of America, a corporation of Delaware Application October 3, 1941, Serial No. 413,486

4 Claims.

Myinvention relates to cathodes for electron discharge devices of the vacuum or gas or vaporfilled types and particularly to cathodes comprising a metal core coated with electron emissive oxides.

For long life and high level emission from an oxide coated cathode there must be a reducing agent in the core metal that will slowly react with the coating material. Experience has shown that where the coating is an alkaline earth, such as barium oxide or a mixture of barium and strontium oxides, the products of decomposition of the coating form a crust or hard layer on the core under the oxide coating. This crust, now commonly referred to as the interface, may be made relatively thin and light in color, or relatively thick and dark in color. Where the interface is dark, the heat radiation from the cathode is high and the thermal eficiency is low.

On the other hand when used in mercury recticathode with a core containing reducing agents which will produce an interface of optimum color and thickness.

Another object of my invention is an oxide coated cathode particularly useful in mercury vapor discharge devices.

The characteristic features of my invention are defined in the following claims and preferred embodiments are described in the following specification.

One electron discharge device, in which my novel cathode may be employed and shown by way of example in the drawing, comprises an envelope l containing an anode 2 and an oxide coated cathode 3. The electrodes may be connected in the usual way to lead-in conductors sealed in the envelope, and the envelope may be evacuated or gas filled. If desired, the envelope may contain the mercury vapor of and be operated as a conventional mercury rectifier.

The cathode of my invention comprises a nickel core of the directly or indirectly heated type coated with barium oxide or mixtures of barium and strontium oxides, and in the core is alloyed or mixed two reducing agents which will produce a relatively thin but continuous interface crust that is light gray in color. One cathode with which good results have been obtained comprises electrolytic nickel containing about .20% silicon and about .15% carbon. After activating the nickel core there appears a thin light gray interface which firmly bonds the oxide matrix to the core and which, tests show, does not objectionably cool the cathode by radiation. This specific alloy is particularly useful in mercury vapor tubes because the interface prevents mercury condensation on and amalgamation or wetting with the nickel. It is found that the silicon primarily controls the color of the interface and that the amount of silicon may be as little as .05% when used in vacuum tubes. In mercury rectifiers less than this amount of silicon is insufficient to produce an interface that will prevent oxide coating from blowing off when operated after long periods of idleness. If the silicon is increased to more than .25 to 30%, too much interface is produced and the oxide coating will peel off.

Carbon, when used with silicon, performs several functions. It acts as a good reducing agent although it leaves no interface. If carbon is used as the only reducing agent, the emission is lower than if silicon is combined with the carbon. Tests show that in tubes operated for long periods. of time in stand-by service, with only the filament lighted, the emission of my cathode remains substantially constant, and does not slump as when nickel-cobalt cathodes containing titanium and iron are used. Carbon, further, is an excellent deoxidizing agent during the melting and casting of the nickel, thus when the silicon is added no further deoxidizing action is required and a controlled amount of silicon is insured in the finished ingot. A further function of the carbon, with the silicon, is that it aids in producing a strong filament, which is especially necessary in maintaining constant interelectrode spacing and in maintaining the spacing of the turns of the filament if wound. If the carbon content in the finished filament is less than about .10% the filament shows mechanical weakness and a decrease in emission, while if the carbon content is more than about .4.0% the nickel becomes hard and unworkable.

Where mechanical strength at high temperature is particularly important, part of the nickel may be replaced by cobalt. Up to 50% of cobalt is used with success in making spiralled fiat ribbon filaments for mercury filled tubes of the type commercially known as the 866/866A. Above this I amount of cobalt the metal becomes difficult to crucible at a temperature of 1560 to 1600" C.

When cobalt is used it is preferably loaded in the bottom of the crucible and'covered with the blocks of nickel. Then pieces of carbon-nickel alloy are added to the melt until the melt becomes quiescent. Sufficient additional carbon is then added to carry the nickel through the rolls and through the swaging, drawing and annealing operations with the final content of about .15%. This additional carbonshould beabout 25% to finally produce nickelwire containing 15% carbon. Alternatively the melt may be deoxidized by stirring with a graphite rod until it becomes completely deoxidized and quiescent and then the specified amount of carbon is added. Therequired amount of silicon, preferably commercial refined silicon, is then added to the-melt in one lump so as to minimize burning and combustion of thesilicon. Finally the manganese is added and at a temperature of about 1535 C. the'melt is poured. into ingots. The ingots'may then be cleaned of surface impurities by grinding or machining and then swaged and drawn to the desired wire or ribbon sizes in accordance with the usual practice of drawing nickel wire.

My improved cathode is strong, firmly bonds the oxide coating to the core and has an interface of optimum color and thickness, and is particularly useful in mercury filled tubes.

I claim:

1. A cathode comprising a core, a layer of electron emissive oxides adhesively bonded to said core, said core consisting of by weight .05 to 25% silicon. and the remainder substantially pure nickel containing .10 to .40% carbon.

2: A cathode comprising a nickel alloy core, electron emissive oxides coated on said core, said alloy consisting of by weight .05 to .25% silicon, .10 to .40 carbon and up to 50% cobalt, the remainderbeing substantially pure nickel.

3. A cathode core, an oxide coating on said core said-core consisting; of by weight about 40% cobalt .20% silicon, .15 carbon and the balance substantially pure. desulphurized' nickel, and having a thinlight gray crust under the oxide coating.

4. A mercury vapor tube comprising a cathode in an envelope containing mercury, said cathode comprising an alloy predominantly of nickel with .05 to'.25% silicon and .10 to .40%- carbon, and a coating on the' cathode of an electron emissive compound.

EMIL GIDEON- WIDELL. 

